1. Field of the Invention
The present invention relates to an LPP (Laser Produced Plasma) EUV (Extreme Ultra Violet) light source apparatus for generating extreme ultra violet light to be used for exposure of semiconductor wafers or the like, in which breakage of a filter for filtering EUV light can be prevented.
2. Description of a Related Art
Recent years, as semiconductor processes become finer, photolithography has been making rapid progress to finer fabrication. In the next generation, microfabrication of 100 nm to 70 nm, further, microfabrication of 50 nm or less will be required. Accordingly, in order to fulfill the requirement for microfabrication of 50 nm or less, for example, exposure equipment is expected to be developed by combining an EUV light source generating EUV light with a wavelength of about 13 nm and reduced projection reflective optics.
As the EUV light source, there are three kinds of light sources, which include an LPP (laser produced plasma) light source using plasma generated by applying a laser beam to a target, a DPP (discharge produced plasma) light source using plasma generated by discharge, and an SR (synchrotron radiation) light source using orbital radiation. Among them, the LPP light source has advantages that extremely high intensity close to black body radiation can be obtained because plasma density can be considerably made larger, that light emission of only the necessary waveband can be performed by selecting the target material, and that an extremely large collection solid angle of 2π steradian can be ensured because it is a point light source having substantially isotropic angle distribution and there is no structure surrounding the light source such as electrodes. Therefore, the LPP light source is considered to be predominant as a light source for EUV lithography requiring power of more than several tens of watts.
FIG. 21 is a schematic diagram showing a configuration of a general LPP type EUV light source apparatus. As shown in FIG. 21, the EUV light source apparatus includes a laser source 101, an EUV generation chamber 102, a target material supply unit 103, and laser beam focusing optics 104.
The laser source 101 generates a laser beam for driving a laser beam to be used for excitation of a target material. The EUV generation chamber 102 is a vacuum chamber in which EUV light is generated. The EUV generation chamber 102 is provided with a window 106 for allowing a laser beam generated from the laser source 101 to pass into the EUV generation chamber 102. Further, a target injection nozzle 103a, a target material collecting tube 107, and an EUV light collector mirror 108 are provided within the EUV generation chamber 102.
The target material supply unit 103 supplies the target material to be used for generation of EUV light through the target injection nozzle 103a as a part of the target material supply unit 103 into the EUV generation chamber 102. A piezoelectric element 103b is provided to the target injection nozzle 103a for vibrating the target injection nozzle 103a during injection of the liquid target material, and thus, droplets 109 of the target material can be formed. The unnecessary material, to which the laser beam has not been applied, of the supplied target materials is collected by the target material collecting tube 107.
The laser beam focusing optics 104 collects the laser beam 110 emitted from the laser source 101 to form a focal point at a laser beam application point 111 in a path of the target material. Thereby, the target material 109 is excited and plasmatized, and EUV light 112 is generated.
The EUV light collector mirror 108 is a concave mirror having an Mo/Si film that reflects light of 13.5 nm, for example, with high reflectivity formed on its surface, and reflects the generated EUV light 112 to the right in the drawing for collection on IF (intermediate focusing point).
The EUV light 112 reflected by the EUV light collector mirror 108 passes through a gate valve 113 provided in the EUV generation chamber 2, and an SPF (spectral purity filter) 114 that removes unwanted light (electromagnetic wave (light) having a shorter wavelength than EUV light, light having a longer wavelength than EUV light (e.g., ultraviolet light, visible light, infrared light, or the like)) of the light generated from the plasma and allows only desired EUV light (e.g., light having a wavelength of 13.5 nm) to pass through. The EUV light 112 collected on the IF (intermediate focusing point) is then guided via transmission optics to an exposure unit or the like. As the SPF 114, for example, a wavelength selective filter described in Japanese Patent Application Publication JP-A-11-126745 may be used.
In this regard, flying matter (called debris) such as fast ions or target material is generated from the plasma, and the flying matter may enter the SPF 114. However, the SPF 114 is thin and the SPF 114 is easily deteriorated and/or broken when the flying matter enters the SPF 114.